The top three most-watched events on the platform, which was launched in 2012, have all come from U.S. matches in this year’s tournament. Team USA’s game against Colombia garnered 164,000 unique viewers, while contests against China and Australia generated 137,000 and 119,000, respectively.

As a comparison, about 96,000 tuned in on Fox Sports Go to see the Packers take on the Seahawks in the NFC Championship football game earlier this year, while 103,000 watched Game 7 of the 2014 World Series on the service. Like many streaming options, consumers need to prove they are pay-TV customers in order to log in.

The streaming viewership is still far less than on traditional TV. According to Nielsen, 5.7 million viewers watched the U.S. team’s match against China on Friday. Through June 23, coverage of the Women’s World Cup has averaged a little over one million viewers per match, compared to 375,000 in 2011, according to Nielsen.

Sports fans aren’t the only ones excited about a summer of soccer. Fox is set to glean about $17 million in sponsorship revenue for the tournament, compared to ESPN’s $6 million ad haul in 2011, CMO Today has reported. That still is far less than the men’s tournament, which generated $529 million last summer.

Clark Pierce, senior vice president of TV Everywhere at Fox Sports, said that while the Fox Sports Go app historically served viewers with the same ads from a linear broadcast, now the ads are “dynamically inserted.” That means spots can be swapped out and better targeted for the online audience.

But streaming is a newer format where viewers can get bombarded with repeated ads or see screens that say a commercial break is in progress. Mr. Pierce said Fox Sports has tried to avoid those pitfalls to “create as good an experience as we can.”

“The World Cup is just a different animal because of the format and flow of the event,” Mr. Pierce said. Soccer has two commercial-free halves with a halftime, making ad-serving an easier endeavor than football, where telecasts cut to commercial breaks every few minutes.

To be sure, it’s easier to walk away and avoid ad time during soccer than it is for football.

U.S. viewership for this summer’s tournament has also likely been helped by time-zone logistics. Canada is hosting this year’s event, while Germany did in 2011.

(21st Century Fox, owner of Fox, was part of the same company as Wall Street Journal-owner News Corp until mid-2013.)